The present invention relates to a harmonic and pulsation reduction circuit used in a multiplex polyphase rectifier circuit and more particularly to an improvement therein which can reduce harmonics and pulsations in a multiplex polyphase rectifier, a branch reactor, an external commutated invertor, respectively having multiplex polyphase rectifier circuits.
As rectifiers have become more popular and the capacity thereof increased in recent years, harmonics from a rectifier presents a problem. Although various countermeasures to solve the problem by making a rectifier polyphase or multipulse or by using a filter have been proposed, they are not desirable as they tend to push cost considerably.
Explanation is given first to a kind of multiples polyphase rectifiers, a double three-phase 12-pulse rectifier having a double three-phase 12-pulse rectifier circuit comprising a combination of a star connection and a delta connection.
FIG. 1 is a circuit diagram showing a double three-phase 12-pulse rectifier. As indicated in the figure, a three-phase bridge rectifier circuit 1 having thyristors T.sub.1 to T.sub.6 as the rectifying device is connected to one of the secondary windings in star connection of a transformer while a three-phase bridge rectifier circuit 3 having thyristors T.sub.7 to T.sub.12 as the rectifying devices is similarly connected to the other secondary windings 4 in delta connection of the transformer. The three-phase bridge rectifier circuits 1 and 3 are connected to each other via an interphase reactor 5a to supply DC current to a load 7 which is connected via a smoothing reactor 6 at the central point A of the interphase reactor 5a. In the figure the reference numeral 8 denotes a primary winding of the transformer.
The waveforms of the voltage V.sub.m which is to be applied to the interphase reactor 5a at the normal operation of the double three-phase pulse rectifier and of the voltage e.sub.d.alpha. between terminals A,B may vary depending on the phase-control angles .alpha. of the thyristors T.sub.1 to T.sub.12 as below:
(1) When the phase control angle .alpha. is as small as almost 0.degree., the waveforms of the voltages e.sub.d.alpha. and V.sub.m will become as shown in FIG. 2(a) and FIG. 2(c). The voltage e.sub.d.alpha. contains pulsation of 12f (f denotes a power source frequency) as indicated in FIG. 2(b) and the voltage V.sub.m becomes a triangular wave.
(2) When the phase control angle .alpha. is as large as almost 90.degree., the waveforms of the voltages e.sub.d.alpha. and V.sub.m will become as shown in FIG. 3(a) and FIG. 3(b). The voltage e.sub.d.alpha. contains sawtooth pulsation of 12f as shown in FIG. 3(a) while the voltage V.sub.m becomes substantially square waveform as shown in FIG. 3(b).
In prior art only feasible method to reduce the pulsation of, in this example, 12f contained in the voltage e.sub.d.alpha. is achieved by twining to said circuit, for instance by using the quadruple zigzag-star connection as mentioned above.